1. Preheat the oven to 190°C. Bring a large pan of water to the boil and add the macaroni. Cook for 10 minutes. Drain.
2. In a small frying pan add leeks and soften. Set aside. Melt the butter in a saucepan and add the flour, beating with a wooden spoon to form a roux.
3. Cook this out for a minute or so, stirring all the time. Add the milk little by little, stirring all the time until sauce is thickened, then add the mustard powder, Remove from heat. Stir in the grated cheese, until combined. Add a few drops of truffle oil.
4. Mix the leeks into the pasta and put in an ovenproof dish. Pour over the cheese sauce and sprinkle over the breadcrumbs.
5. Put in the oven for 25 minutes, until bubbling and crispy on top. To make the salad dressing mix all the ingredients for the dressing and the chives in a jam jar.
6. Adjust to taste then pour over the lettuce.